Microsoft's decision to switch on DNT (Do Not Track) by default in IE10 (Internet Explorer 10) will have to be rethought if the company wants to claim it supports the developing privacy standard.

On Wednesday, the W3C (World Wide Web Consortium) standards organization reached a compromise on some aspects of DNT, the browser feature that signals whether a user wants online advertisers and websites to track his or her movements.

The new draft of the standard, which may be months from passing in final form, explicitly bars browsers from setting DNT on by default. "An ordinary user agent MUST NOT send a Tracking Preference signal without a user's explicit consent," the draft reads (download PDF).

That seemed squarely aimed at Microsoft.

Last week the company announced with some fanfare that IE10, the new edition to be bundled with Windows 8 and its tablet offshoot Windows 8, and to be made available as an upgrade on Windows 7, would set DNT on by default.

Microsoft's chief privacy officer, Brendon Lynch, made it crystal clear. "We believe turning on Do Not Track by default in IE10 on Windows 8 is an important step in this process of establishing privacy by default, putting consumers in control and building trust online," Lynch wrote in a May 31 blog.

But the W3C group that's been hammering out DNT disagreed and said flatly that while Microsoft is perfectly free to do what it wants, it cannot call IE10 DNT compliant if it continues down its on-by-default road.

"Microsoft IE, as a general purpose user agent, will not be able to claim compliance with DNT once we have a published W3C Recommendation," Aleecia McDonald, a researcher at Stanford's CIS (Center for Internet and Society) and a part-time employee of Mozilla, said in a summary of a Wednesday conference call. Mozilla supports her work as co-chair of the W3C effort on DNT.

"As a practical matter, they can continue their current default settings, since DNT is a voluntary standard in the first place. But if they claim to comply with the W3C Recommendation and do not, that is a matter the FTC (and others) can enforce," McDonald said.

Mozilla, in fact, had staked out its position earlier.

"At its foundation, DNT is intended to express an individual's choice, or preference, to not be tracked," said Alex Fowler, who leads Mozilla's privacy and policy work, in a blog post written the same day Microsoft said IE10 would have the signal on by default. "It's important that the signal represents a choice made by the person behind the keyboard and not the software maker, because ultimately it's not the browser being tracked, it's the user."

Firefox, Fowler continued, supports DNT, but leaves it in the "off" position which lets -- or makes, depending on the viewpoint -- the user choose. "For DNT to be effective, it must actually represent the user's voice," Fowler said.

Either Microsoft or the W3C group will have to blink. And it doesn't sound like the W3C will be the one to back down.

"This is as far as we're willing to go," said Jonathan Mayer, also a researcher with the Stanford CIS. He was referring to the group he called "privacy-leaning parties" who, he said, have compromised as much as they're going to.

Mayer and others argued that IE10, or any browser for that matter, should be allowed to set DNT as on by default, but gave in to keep the discussions on the standard moving. "Absolutely, browsers should be able to set a default. They set all kinds of defaults. But we, and I mean me and many of the others on the privacy-leaning side, did this to show how committed we are to DNT."

Mayer is also one of the two researchers who came up with the HTTP header concept that browsers use to communicate a user's DNT choice desire to websites.

An alternative to an automatic-on setting would be for a browser to display a dialog box the first time it's run, asking the user how he or she wants to set DNT.

At least two other browser vendors -- Mayer declined to name them -- have expressed interest in that concept, he said. "They're seriously thinking how they can position themselves against IE," Mayer said.

According to Mayer, that "first-run" option has been discussed by the DNT group, but not yet decided.

Also still up in the air is what websites and advertisers will be bound to do when they recognize IE10, assuming Microsoft does not change its mind on DNT and the default setting it has planned.

"We don't have agreement on what the ramifications are. Can ad networks ignore a tracking request from IE10?" Mayer said. "Google and Yahoo and Adobe said they should be able to ignore the header from IE10, but Mozilla and Apple have said that ad networks should not ignore it."

Microsoft was not available for comment on the W3C draft specification that would bar it from advertising IE10 as compliant with DNT.

Mozilla also did not reply to questions related to the ongoing discussions of the W3C group.

Microsoft has not said when it will ship a final version of IE10 -- apparently that will not happen until Windows 8 launches later this year -- but enabled the DNT signal with the sixth early build of the browser that was bundled with Windows 8 Release Preview. That sneak peek debuted a week ago today and is available for free downloading from Microsoft's website.